Goal: To understand Electrostatics

1
Goal To understand Electrostatics

• Objectives
• Understanding what charges are.
• Knowing how to produce a charge.
• How to calculate an electric field
• Electric Force
• Electric Potential
• Other random stuff

2
What is charge?

• For the most part, charge is a measure of how
  many protons or electrons you have somewhere.
• Charge is measured in units of Coulombs (C).
• An elementary charge from a proton or electron
  has magnitude of 1.602 10-19 C.
• Like charges repel. Opposite attract.
• Charges can move.

3
How do you get charge?

• 1) Rubbing (static electricity)
• 2) Induction (charge obtained from a changing
  magnetic field)
• 3) Conduction (moving charge along a wire)

4
Electric Field

• Suppose you wanted to know where the water would
  flow when it rains.
• How would you do that?

5
Fields

• Fields are just a listing of possible potential
  at any given point.
• For rain you look at the Gravitational Field
  which is just a fancy way of saying the
  topography.
• Water will want to flow downwards.
• We can do the same with electric fields.

6
Electric Field

• The Electric Field is just a measure of the
  electric topography.
• Since protons repel each other you can think of
  the protons as hills.
• The electrons would be pits or valleys.
• The elevation of some point near some charges
  would depend on the distribution of charges (much
  like your elevation depends on where you are
  compared to the hills and valleys).
• Units are in N / C.

7
Calculating the Electric Field

• First lets do it for just one charge.
• For one charge the equation is pretty
  straightforward
• E -k charge / (distance distance)
• k is a constant, and is 9 109 N m2 / C2
• There is another equation for the electric field
  we will use later.

8
Force translation

• the Electric field is a topography of electric
  charges around you.
• At any point the electric field is just a sum of
  the topography from each charge.
• For each charge E -qk / r2
• How would this translate to a force?

9
Ball downhill

• If you have a gravitational topography a ball
  will want to roll downhill.
• That is it will roll from a high elevation to a
  low one or a high field to a low one.
• The same is true of electric fields.
• A positive charge will want to move to a lower
  electric field.
• A negative charge will do the opposite and will
  want to move up to a higher valued electric field
  (moving uphill).

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Now for the math

• The force on a charge is
• F q E
• Where q is the charge the force is being applied
  to and E is the electric field that charge q is
  located at.
• Much like for gravity that F m g on the
  surface of the earth.

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If we add in E

• If we have 2 charges called q1 and q2 then the
  force is
• F q1 E, but E -q2 k / r2
• So, F -q1 q2 k / r2
• (k is the same constant we had before)
• Notice this is almost the same as the
  gravitational force where
• F -G M1 M2 / r2

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Which force is stronger?

• During the next 5 min break think about which of
  the following is a stronger force?
• Electric force F -q1 q2 k / r2
• Or gravity F -G M1 M2 / r2

13
Electric Potential Energy

• Electric Potential energy is similar to
  gravitational potential. It is the potential
  energy between any two charges.
• Energy is a force times a distance, so if you
  multiply the Electric Force times a distance (I
  am oversimplifying a little here) you get the
  Electric Potential.
• U k q1 q2 / r (and is in units of Joules)
• Note how similar it looks to the equation for the
  force.

14
Direction?

• Other than calculating distances between charges
  will the directions matter?
• Why or why not?
• NO! Potential energies are SCALER values!

15
Insulators

• Insulators (like insulation for the house) is a
  material that wont let charges move through it.
• Wood would be an insulator
• What is a real world use for an insulator?

16
Conductor

• Materials that help the flow of charges are
  conductors.
• Ideally, you want to make a wire out of a good
  conductive material.

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What about in between

• There are materials that can act as both
  conductors and inductors.
• These are called semi-conductors.
• These form vital electronic parts such as
  transistors (which revolutionized the electronics
  industry)

18
Super strength conductors

• The only draw back to conductors is that they
  heat up (and that means you are loosing power).
• If you send power across a long power line, you
  loose energy, and that looses you MONEY!
• But what if you could build a conductor that
  didnt give any resistance to the flow of charge?
• You would have a superconductor

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Superconductors

• Typically superconductors are made of materials
  that when cooled to VERY low temperatures (-300
  to 400 F) they allow charges to flow freely.
• There is a lot of uses for this, but it is not
  very practical unless you can build them to
  operate at temperatures that are much closer to
  room temperature.

20
Saving charges

• Collecting charges requires a device called a
  capacitor.
• This is usually a pair of sheets separated by a
  small distance on which you store charge.
• The negative electrons collect on one sheet.
• One the opposing sheet the electrons are repelled
  which can then flow through the rest of the
  circuit leaving only a positive charge.

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Flood!

• However, there is a limit.
• If you try to collect too much charge, you get a
  discharge.
• The electrons flow over like water flowing over a
  filled to capacity dam.

22
Dielectrics

• As you read in the book (hopefully) you can
  shield charges.
• One way is by filling the center of the capacitor
  with a substance.
• This substance is a dielectric.
• The type of material will determine what fraction
  of the charge the other side actually sees.
• Water, for example, allows you to collect 80
  times more charge than air.

23
Conclusion

• 1) We learned how to find the Electric Field and
  electric force
• 2) We have found how to do the electric
  potential.
• We have seen applications for electrostatics such
  as conductors and inductors.
• We have seen how to store charge.